Thermoplastic polymers such as polyethylenes, polyamides or blends thereof are good electrical insulators and are easy to employ. They are used to make cases and electrical connectors and also cable coatings. Electrical systems may be the origin of short circuits and may catch fire, and they may also be contacted with a flame and hence catch fire and propagate the fire along the cable routes. A variety of additives exist for making these materials nonflammable, some of said additives being based on halogenated products, and others being halogen-free. The use of halogenated additives is increasingly being banned for reasons of ecotoxicology and toxicology (toxicity and corrosiveness of the vapors emitted during fires).
According to the UL 94 flame propagation tests in accordance with the standard ISO 1210, the V0 classification is the best classification, corresponding to a material which is difficult to ignite and does not produce flaming droplets during the test.
For the V1 classification, the material is easier to ignite but does not produce flaming droplets during the test. As far as the V2 classification is concerned, in addition to the greater ease of ignition than for V0, flaming droplets may be produced during the test.
Materials which are even more flammable are given the label NC (not classified).
Patent EP 629 678 describes thermoplastic alloys comprising blends of polyamide and polypropylene, to which a zeolite and ammonium polyphosphate (dehydrating agent) are added to make them flame retardant. The blends of polyamide and polyolefin are composed (by weight) of 57% of polyamide 6 (PA 6), 33% of a propylene homopolymer, and 10% of a polypropylene grafted with maleic anhydride and then condensed with a monoamino polyamide oligomer. 30 parts (by weight) of ammonium polyphosphate (APP) and one part of zeolite were added to 69 parts of the aforementioned mixtures, after which test specimens with a thickness of 3.2 mm were produced by injection molding. These test specimens were subjected to the UL 94 flame propagation tests in accordance with standard ISO 1210, the classification obtained being V0. V0 is the best classification according to this test. These blends, which are composed of a PA matrix, do not contain phosphorus plasticizer, and there is no indication of any mechanical property such as, in particular, elongation at break.
Patent EP 704 489 describes compositions composed of a polyamide matrix containing, dispersed therein, crosslinked polyolefin nodules and flame retardants selected from magnesium hydroxide, decabromodiphenyl ether, melamine cyanurate, and pentaerithritol. These compositions are useful for sheathing electrical cables. They contain no polyphosphates, no phosphorus plasticizer, and no zeolite.
However, the fire resistance performance of these prior-art compositions is generally obtained at the expense of the ductility of the materials (considerable loss in elongation at break, fragile impact behavior at ambient temperature). Moreover, the thermal stability of these materials is found to be inadequate. By thermal stability is meant the retention of mechanical properties (and more particularly elongation at break) after various forms of thermal aging (for example, 1 week at 120° C. in hot air).
The Applicant's document EP 1 375 594 describes compositions rendered flame retardant without halogen compounds and comprising, by weight, the total being 100 parts:                50 to 75 parts of a blend of polyamide (A) and polyolefin (B), comprising (i) a high density polyethylene (HDPE) and (ii) a blend of a polyethylene (C1) and a polymer (C2) selected from elastomers, very low density polyethylenes, and ethylene copolymers, the blend (C1)+(C2) being co-grafted by an unsaturated carboxylic acid,        25 to 50 parts of a mixture comprising:        0.1 to 48.8 parts of a flame retardant, of ammonium polyphosphate type,        0.1 to 30 parts of a phosphorus plasticizer,        0.1 to 10 parts of a zeolite.        
These compositions exhibit a V0 or V1 classification according to the UL 94 test, when the fire test is conducted on test specimens with a thickness of 1.6 mm, and their elongation at break, measured according to ISO R 527-1B, exceeds 100%.
The blend of (A)+(B) includes 60% to 70% by weight of polyamide (A), which constitutes its matrix.
These formulations contain a phosphorus plasticizer, the atomic element phosphorus being known to contribute to the flame retardance of materials. On the other hand, these plasticizers, being small molecules, have an observed tendency to depart the resulting material (to exude), which is unacceptable for the applications intended. New formulations have therefore now been found which are not plasticized (and hence are nonexuding) but which nevertheless possess the same level of flame retardance.
Moreover, the halogen-free flame retarding of functionalized polyolefins is well known, especially for cabling applications. It involves introducing very large amounts (60% to 65% by weight) of hydrate fillers such as aluminum trihydrate (ATH) or magnesium dihydroxide (MDH) in order to provide the expected levels of flame retardance. The transposition of flame retardance formulas of this kind to PA-grafted ethylene-alkyl (meth)acrylate-maleic anhydride copolymer materials, of the type of those described in document WO 02/28959 to the Applicant, which relates to a blend of a graft polymer containing polyamide blocks and composed of a polyolefin backbone and on average at least one polyamide graft, and of a flexible polyolefin, leads to a catastrophic loss of mechanical properties in the composition, especially as regards elongation at break (value <100%).
Moreover, a PA graft copolymer of this kind exhibits no flame propagation resistance (UL94 test: NC=not classified for Lotader® 3410-g-PA6).
However, certain target applications for these materials (cabling, electrical and electrotechnical components, automotive) require a combination of such properties.